Research on antibiotic resistance encoding plasmids in bacteria has led to the view that extrachromosomal elements provide a means for rapid evolution and frequent transfer of genetic information between bacterial lineages. However, little is known about the population dynamics and evolution of most naturally occurring plasmids. Do they represent an extension of the bacterial genome, serving as transport vehicles for bacterial genes, or are they parasites with a biological integrity distinct from that of their hosts, and thus constitute separate species? In this proposal colicin plasmids of Escherichia coli are employed as a model to investigate the forces that influence the population dynamics and molecular evolution of plasmids and thus address the "plasmids as species" concept. A combination of restriction mapping and DNA sequencing will be used to assess the levels and patterns of molecular polymorphism for a representative sample of colicin plasmids and their colicin genes. These data will provide information on the stability of plasmid lineages, the extent of horizontal transfer of plasmids in E. coli and the rates and patterns of evolution for plasmid determined genes. The availability of similar data for chromosomal genes from the same collections of E. coli employed here allow a comparison of rates and patterns of evolution between chromosomal and plasmid determined genes.